<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        #parent{
            width: 350px;
            height:300px;
            border: 1px solid red;
        }
        #s1,#s2,#s3{
            width: 100px;
            height: 100px;
        }
        #s1{
            background-color: green;
            float: left;
        }
        #s2{
            background-color: blue;
            float: left;
            margin-left: 10px;
            margin-right: 10px;
        }
        #s3{
            background-color: yellow;
            float: left;
        }

        /*    浮动*/


    </style>
</head>
<body>


<div id="parent">
    <div id="s1">1</div>
    <div id="s2">2</div>
    <div id="s3">3</div>
</div>
</body>
</html>